Answer: Congratulations on your new adventure! I'm considering building one of those myself--our season is so short here in Vermont!
One important thing for you to consider is how you will ventilate your greenhouse. It can get very hot, very quickly, in an enclosed structure. If you're around during the day to monitor this, you can do it manually--just set a thermometer inside and when it gets above about 70F, crack open the top. If you aren't available to do this daily, then you might want to invest in a solar-powered vent opener, which will automatically open and close a vent at whatever temperature you set it at.
I think the best use of your greenhouse will be as a season extender. You'll be able to harden things off earlier in the spring, and will be able to grow lots of seedlings outdoors, instead of crowded under fluorescent lights indoors. I would hesitate to start much in the greenhouse in the summer, because temperatures in there can get so hot. Again, if you can closely monitor temps., then you should be OK. But most seedlings grow best at temps of 65-70 or so--hot temperatures can cause leggy growth and stunt the plants. And cole crops like broccoli and cabbage in particular like cool temperatures. (As do other good fall crops like greens, peas, lettuce, and spinach.)
You might want to experiment with methods to keep the greenhouse cool in the summer months--such as fans or shade cloth--before placing plants in there. Charley's Greenhouse Supply, (1569 Memorial Hwy, Mount Vernon, WA 98273, ph# 800-322-4707) may have some useful products and information for you. Two good reference books are: Greenhouse Gardener's Companion, by Shane Smith (Fulcrum, 1993) and Greenhouse Gardening, by Miranda Smith (Rodale, 1985). They'll help you figure out solutions to any problems that come up--for example in pest management--and give you some guidelines about choosing crops and planting times.
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